In recent years, a battery using a carbon material such as graphite as a negative electrode and using a lithium-containing metal oxide such as LiCoO2 as a positive electrode, has been proposed. This battery is a so-called rocking chair battery such that after it is assembled, lithium ions are supplied from the lithium-containing metal oxide as the positive electrode to the negative electrode by charging, and the negative electrode lithium ions are returned to the positive electrode by discharging. This battery is called a lithium ion secondary battery since no lithium metal is used for the negative electrode but only lithium ions are involved in the charge and discharge, and the battery is distinguished from a lithium battery using lithium metal. This battery is characterized with a high voltage, a large capacity and high safety.
Further, as concern for environmental problems is increasing, storage system for clean energy by solar power generation or wind power generation, and power sources for electric automobiles and hybrid electric automobiles which replace gasoline-powered vehicle, have been actively developed. Further, along with the tendency to high quality and high functionality of on-vehicle apparatus and equipment such as power windows and IT devices in recent years, a new power source has been required in view of the energy density and the output density.
As a storage device to be used for such an application which requires a high energy density and high output characteristics, attention has been paid to a storage device called a hybrid capacity comprising a combination of storage principles of a lithium ion secondary battery and an electric double layer capacitor in recent years. As one example, an organic electrolyte capacitor has been proposed (for example, Patent Document 1) in which as a negative electrode, carbon material obtained in such a manner that lithium ions are preliminarily made to be absorbed and supported (hereinafter sometimes referred to as doping) by a carbon material capable of absorbing and releasing lithium ions by a chemical or electrochemical method to lower the negative electrode potential, is used, whereby the energy density can be significantly increased.
Such an organic electrolyte capacitor is expected to show high performance, but has drawbacks such that when lithium ions are preliminarily supported on the negative electrode, the supporting requires a very long time, and it tends to be difficult to make lithium ions be uniformly supported by the entire negative electrode. Particularly, for a large-size, large capacity cell such as a cylindrical apparatus having electrodes wound or a rectangular battery having a plurality of electrodes laminated, it has been considered to be hardly used practically.
To solve such problems, an organic electrolyte battery has been proposed (for example, Patent Document 2), wherein each of a positive electrode current collector and a negative electrode current collector has pores penetrating from the front surface to the back surface, a negative electrode active material is capable of reversibly supporting lithium ions, and lithium ions are supported by the negative electrode by electrochemical contact with lithium metal disposed to face the negative electrode or the positive electrode.
In the organic electrolyte battery in which the electrode current collector has pores penetrating from the front surface to the back surface, lithium ions can move from the front surface to the back surface of the electrode without being blocked by the electrode current collector. Thus, even in a storage device having a cell structure with a large number of electrodes being laminated, it is possible to make lithium ions be electrochemically supported by not only a negative electrode disposed in the vicinity of lithium metal but also a negative electrode disposed distant from lithium metal, via the through-pores.
Further, Patent Document 2 discloses a cell structure of the above organic electrolyte battery using a positive electrode and a negative electrode. FIG. 8 illustrates the cell structure of the above battery wherein lithium metal is provided on the lower portion of an electrode laminate unit. As shown in the drawing, in this cell, positive electrodes 1 formed on a positive electrode current collector 1a and negative electrodes 2 formed on a negative electrode current collector 2a are alternately laminated with a separator 3 interposed therebetween to constitute an electrode laminate unit 6, each of the upper and lower outermost portions of the electrode laminate unit 6 is a negative electrode 2′, and lithium metal 4 is disposed to face the lower negative electrode 2′. As the electrodes, the electrodes 1 and 2 in the main portion of the electrode laminate unit 6 are ones having electrode layers on both sides of the current collectors 1a and 2a, and the negative electrodes 2′ disposed at the outermost portions of the electrode laminate unit 6 are ones having an electrode layer only on one side. Even when the outermost portion of the electrode laminate unit 6, to which no lithium metal 4 is disposed, is a positive electrode, similarly one having an electrode layer only on one side is used for this positive electrode.
As mentioned above, in a conventional cell, an electrode having an electrode layer only on one side of a current collector is used for each of the outermost electrodes of the electrode laminate unit constituting the cell. The reason will be explained with reference to FIG. 9. FIG. 9 is a schematic cross section illustrating the outermost portion of a conventional organic electrolyte battery. As shown in the drawing, an electrode layer 14 formed on one side “a” of a current collector 13 (having through pores) has a counter electrode layer 15, whereby the electrode layer 14 undergoes charge and discharge together with the counter electrode layer 15. However, since it has no counter electrode layer on the outside surface b (outermost portion), if an electrode layer 14′ (imaginary line) is formed on the outside surface b, this electrode layer 14′ also undergoes charge and discharge together with the counter electrode layer 15 on the one side “a” via the through pores, and thus a load corresponding to the electrode layer 14 and the electrode layer 14′ on both sides of the current collector 13 is applied to the counter electrode layer 15. Therefore, in a case where the outermost electrode is a positive electrode having electrode layers on both sides of the current collector 13, a load corresponding to the positive electrode layers on both sides is applied to one side of the counter negative electrode, whereby the negative electrode potential tends to be low, and lithium metal tends to be deposited, thus causing short circuit. In a conventional organic electrolyte battery, in order to avoid the above phenomenon, the outermost electrode portion is an electrode having no electrode layer 14′ on the outside surface b i.e. having an electrode layer 14 only on one side “a” of the current collector 13.
Patent Document 1: JP-A-8-107048
Patent Document 2: WO98/033227